1. Field of the Invention
This invention relates to ring laser gyroscopes, and more particularly to ring laser gyroscopes having magnetic sources external to the bore cavity of the closed path defining the gyroscope.
2. Description of the Related Art
Since its introduction in the early 1960's as a laboratory experiment, the ring laser gyroscope has been commercially developed as a logical replacement for the mechanical gyroscope for use in all manner of inertial guidance systems. Heretofore, the basic two mode ring laser gyroscope has been developed which has two independent electromagnetic wave modes oscillating in an optical ring cavity. When the ring is stationary, no rotation is ideally indicated. As the ring cavity is rotated about its central axis, the counter-rotating waves interact with one another so that a beat frequency is developed. A linear relationship between the beat frequency and the rotation rate of the cavity with respect to the inertial frame of reference may be established. Ideally, the rotation rate is proportional to the beat note. In this manner a gyroscope is theoretically produced having no moving parts.
In practice, however, the two mode laser gyroscope often must be mechanically dithered to keep the counter rotating travelling waves from locking at low rotation rates. For more information on planar gyroscope two mode lock in, please see Laser Applications, edited by Monte Ross, pp. 133-200 (1971). In an effort to solve this lock-in problem, non-planar ring cavities have been designed containing more than one pair of counter rotating modes. These multi-oscillator ring laser gyroscopes have been developed to achieve the goal of an accurate all optical gyroscope having no moving parts. However, these multi-oscillator ring laser gyroscopes require the use of a non-reciprocal polarization rotation device (such as a Faraday rotator) to achieve the splitting of the light within the ring cavity into two pairs of counter rotating modes. Generally, the multi-oscillator ring laser gyroscope is divided into a pair of right circularly polarized and left circularly polarized waves. The right circularly polarized waves are split by the Faraday rotator into clockwise and anti-clockwise modes. Likewise, the left circularly polarized waves are split by the rotator into clockwise and anti-clockwise modes. For a full discussion of the multi-oscillator ring laser gyroscope, please see LASER HANDBOOK (vol. IV) edited by M. L. Stitch (1985), pp. 229-332. A non-planar configuration comprising at least four mirrors and a non-reciprocal Faraday rotator is described in Smith, U.S. Pat. No. 4,548,501 issued Oct. 22, 1985. In such a non-planar configuration, reciprocal rotation is accomplished by the non-planar geometry of the multi-mode ring laser gyroscope. The out-of-planeness geometry in a folded rhombus ring laser gyroscopes provides the necessary the reciprocal splitting into left and right circularly polarized beams. However, the clockwise and anti-clockwise component of each circularly polarized beam are essentially locked at low rotation rates. In order to further split the right and left circularly polarized beams into their clockwise and anti-clockwise frequency components, a non-reciprocal rotator means such as a Faraday rotator is used. Since the left and right circularly polarized sets of beam modes are widely separated in frequency, the multi-mode ring laser gyroscope avoids the problem of mode lock-in common to two mode ring laser gyroscopes.
Critical to the success of non-reciprocal splitting in a multi-oscillator ring laser gyroscope is the need to provide a uniform low gradient magnetic field inside the Faraday rotator disk. Alternatively, an all optical out-of-plane geometry ring laser gyroscope having no intra-cavity elements for either reciprocal or non-reciprocal splitting is disclosed in a patent application assigned to the common assignee of this application and entitled "SPLIT GAIN MULTI-MODE RING LASER GYROSCOPE AND METHOD" by Graham Martin, Ser. No. 115,018, dated Oct. 28, 1987, now U.S. Pat. No. 5,386,288 (placed under secrecy order May 17, 1988). As is disclosed in detail in the cited co-pending application, this alternative ring laser gyroscope uses high and uniform magnetic fields to achieve a split of the gain curve into Q and (Q+1) modes so as to achieve a desired effect that is equivalent to Faraday rotation. This split of the gain is achieved by the use of high power, highly concentrated, magnetic fields properly positioned along the bore cavity of the ring laser gyroscope.
Heretofore, the multi-oscillator ring laser gyroscope and the split gain ring laser gyroscope have applied axial magnetic fields along a segment of the closed path formed by the bore cavity by use of cylindrical, hollow magnets positioned parallel and around the bore segment or within the segment. In the multi-oscillator ring laser gyroscope, a Faraday rotator glass was typically concentrically mounted within a tubular axially directed magnet, the entire assembly being "musket-loaded" into the bore cavity where the Faraday rotator is aligned and positioned in the optical pathway. This is a difficult and time-consuming procedure. The "musket-loading" of the Faraday rotator and magnet assembly must not scratch the side wall of the bore cavity. Such "musket-loading" assembly of the Faraday rotator of the multi-oscillator ring laser gyroscope was difficult to assemble. Also it is difficult to place the magnets within the cavity bore separate from the evacuated region.
In the case of the split gain multi-mode ring laser gyroscope, an entire leg segment of the monolithic glass block from which the ring laser gyroscope is manufactured must be carved out to accommodate a hollow cylindrical magnet which is positioned in parallel to the closed pathway and around said pathway. This design requires severe and costly machining of a segment of the closed path and bore cavity of the ring laser gyroscope in order to accommodate the placement of the cylindrical magnet about the segment. The split gain ring laser gyroscope requires precision machining in order to accommodate the placement of a permanent magnet of a cylindrical form around the outer portion of the body of the ring laser gyroscope along a segment of its closed path.